1. Field of Invention
This invention relates to implantable electrodes and more particularly to a cardiac pacing lead distal tip electrode.
2. Description of Related Art
Pacemaker leads are used to electrically connect a cardiac pacemaker pulse generator to heart tissue to be stimulated. For example, endocardial type leads which are inserted into a vein and then guided into the desired heart cavity include at their distal end an electrode tip designed to contact the endocardium or the tissue forming the inner lining of the heart. These leads, connected to a pacemaker, are commonly used for both sensing electrical signals produced by the heart and providing pacing stimulation.
The electrical pacing signal that is delivered to the cardiac muscle must be of sufficient magnitude to depolarize the excitable cells that are adjacent to the electrode tip. The electrode size and shape, tissue conductivity, and the distance separating the electrode tip from the excitable cells are factors in determining the stimulus threshold. Many of these factors are highly determined by the geometry and material composition of the electrode.
The duration or battery life of a pacemaker is, in part, dependent on the current drain that is used in stimulating the cardiac muscle. This current drain is determined by the programmed voltage, pulse width, the rate of the pacemaker stimulator and the pacing impedance presented to the pulse generator. It is important to note that improvements in pacemaker longevity due to increased pacing impedance are not dependent upon reprogramming the pacemaker in any manner.
The pacing impedance is a function of the macroscopic surface area of the electrode. As it is optimal to have a high pacing impedance, most modern pacing electrode designs strive for a reduced area stimulus electrode. Thus, small diameter electrodes will reduce the stimulus current necessary to pace the heart and will extend the life of the pacemaker. Electrodes having very small tip surface areas, in some designs, are problematic in that the small surface area or sharp point can increase the chance of the electrode perforating the ventricular wall, which can lead to blood loss into the pericardial sack. In addition, small tip electrodes are also very sensitive to implantation angle and can demonstrate marked stimulus threshold variability during occurrences of lead micro-dislodgement due to the very uneven surface structure of the endocardial wall. At times the sensitivity to stimulus threshold with micro-dislodgment can cause exit block or complete loss of cardiac stimulation.
It should also be noted that electrodes having very small stimulus areas are prone to generate large polarization artifact signals. These voltage signal distortions are inefficient in that they take energy away from stimulation of the cardiac tissue. More importantly, these artifact signals can present problems to the pacemaker in sensing the following heart activity. One method to reduce this artifact is to increase the microscopic surface area of the electrode, while keeping the macroscopic surface area fixed. This microscopic surface area is the sum of all the microscopic cracks, crevices and indentations on the surface of the electrode.
The electrode must also provide a means for sensing the electrical activity or signal of the heart. The ability to efficiently detect heart activity is directly related to the sensing impedance of the electrode. Optimal sensing occurs with low sensing source impedance electrode designs. Thus large macroscopic surface area electrodes are desired for sensing.
The pacing, or stimulating, threshold is a measurement of the energy required for a voltage pulse to initiate a contraction in the heart tissue. The stimulus threshold typically rises after implantation of an electrode since there is an increase in the spacing between the electrode tip and the excitable cardiac tissue. This is a typical foreign body tissue healing response to the electrode tip and this healing response includes the generation of a fibrous capsule around the electrode tip. Lower stimulus thresholds have resulted from electrode designs with a porous structure at the distal electrode end. Optimal porous structures appear to minimize the initial foreign body reaction and hasten the subsequent healing response to the pacemaker lead tip electrode.
Thus, a considerable design challenge in current state-of-the-art electrodes is the optimization of the electrode surface area, geometry and porosity. High pacing impedance is optimally achieved by low macroscopic surface area electrode geometry. Low polarization losses are optimally achieved by a high microscopic surface area electrode geometry. Low sensing source impedance requires large macroscopic surface area electrode geometry. Low sensitivity to micro-dislodgement requires large macroscopic surface area electrode geometry. The design outcome is always a compromise between the opposite desired extremes. Recent devices utilize various types of surface coatings or metal surface enhancements (e.g., iridium oxide). These surface changes increase the microscopic surface area while keeping the electrode macroscopic surface area relatively the same. These surface enhancements help reduce the polarization losses for a given tip geometry but do not fully solve the design tradeoff concerns on the electrode surface.
An electrode tip design, taught in U.S. Pat. No. 3,476,116 by Parsonnet et al., utilizes an electrode tip with a fluid filled cavity. Within this cavity is a high surface area electrode. The fluid filled cavity is isolated from the tissue to be stimulated by an electrically insulating material containing a small aperture. This electrode tip design has, in effect, a large electrode surface area which lowers the polarization losses. The tissue to be stimulated however perceives a very small surface area due to the small aperture, resulting in high tip to tissue impedance. This design performed reasonably well short term, however the long term or chronic performance was shown to be compromised. The small aperture of the Parsonnet design was highly sensitive to lead movement due to micro-dislodgment which changed the interface between the tissue and the small aperture. This aperture dislodgment caused high stimulation voltage thresholds in some patients and in extreme cases caused total electrode exit block which is a complete failure to stimulate.
A modified Parsonnet design was disclosed by F. Hoffmann in an article entitled xe2x80x9cStimulating Electrode With Low Energy Consumptionxe2x80x9d (Medical and Biological Engineering, September 1973, Pg. 659-660). This proposed design added additional holes or apertures to the original Parsonnet design. The sensitivity of the tip to tissue interface was effectively reduced, however consistent and stable chronic pacing thresholds were still not obtained.
A similar electrode tip design is disclosed in U.S. Pat. No. 5,282,844 to Stokes et al. To achieve low polarization losses, Stokes et al. teach the use of a fluid filled cavity containing an electrode with a large surface area, similar to that of Parsonnet et al. Low stimulation voltage thresholds are achieved by the use of a cavity sheath with a small aperture, again similar to the Parsonnet design. To overcome the chronic increase in stimulation voltage, the Stokes design incorporates a steroid eluting device contained within the bodily fluid filled cavity. The steroid elution alters the results of the reaction to the foreign body response at the electrode tip to tissue interface and results in low chronic stimulation voltage thresholds.
In U.S. Pat. No. 4,011,861, Enger teaches the use of an electric terminal, with a porous outer sheath. The porous sheath encourages the ingress of blood vessels without the production of a fibrous tissue interface which would result in high stimulation voltages. The large number of pores result in a large number of sites of current loss with no areas of high current density nor a marked increase in stimulus pacing impedance.
MacGregor teaches in U.S. Pat. No. 4,281,669 a high surface area, sintered metal electrode tip, incorporating an outer porous polymeric covering. The pores provide for an improved tissue ingrowth structure at the tip. The high surface area sintered metal electrode provides low polarization losses. Similar to Enger, the large number of pores of MacGregor result in no areas of high current density for stimulation.
In U.S. Pat. No. 5,090,422 to Dahl et al., an electrode sheath is disclosed. Dahl et al. teach the use of a porous polymeric sheath, which when impregnated with bodily fluids, becomes electrically conductive. U.S. Pat. No. 5,609,622 to Bush also discloses a porous polymeric sheath. This polymeric sheath has a pore size of less than 10 microns for the purpose of precluding tissue attachment which facilitates removal of the lead after chronic implantation. The porosity also allows bodily fluids to impregnate the sheath thereby allowing electrical energy to pass through the sheath. The porous polymeric sheaths disclosed in Dahl et al. and Bush result in a large number of very small sites of current loss with no areas of high current density nor a marked increase in stimulus pacing impedance.
The present invention provides a layered electrode having an electrically conductive material, covered by one or more layers, wherein the electrode provides high pacing impedance, a low chronic stimulation voltage threshold and low post pacing polarization (artifacts. Specifically, the present invention is an electrode comprising an electrically conductive material which is covered or substantially covered by a layer of substantially electrically insulating material having at least one macroporous perforation (or aperture) therethrough, and a microporous cover over the perforation. The at least one macroscopic perforation provides a high current density path while the microporous cover is permeable to electrically conductive body fluids which allow current to flow through the cover. Preferably the microporous cover simultaneously prevents tissue ingrowth into the at least one perforation.
In a preferred embodiment, the microporous layer is provided as two layers in the form of an external microporous layer having a pore size appropriate to promote tissue attachment to that layer by allowing tissue to grow into the pores of that layer, and an inner cell exclusion layer with pores adequately small to restrict or entirely prevent cell ingrowth. Both layers together are permeable to body fluids. In another preferred embodiment which may be used with either the single or two layer microporous cover, the electrically conductive material of the electrode is in the form of an electrically conductive component provided with a surface of large area such as a porous metal, powdered metal, sintered metal, or any other means of enhancing the surface area of the electrically conductive component in order to enhance the charge transfer between the electrically conductive component and electrically conductive body fluids. The means of enhancing surface area of the electrically conductive material may involve the addition of one or more layers to the surface of the electrically conductive material.
These multiple layers, in concert, can provide good biocompatibility, electrode tip anchoring to the tissue to be stimulated, prevention of cell proliferation into the subsequent layers, one or more localized high current density stimulation sites, a high pacing impedance due to an effectively small macroscopic surface area electrode, and a low post pacing polarization artifact.